<!DOCTYPE HTML>
<canvas width="100" height="100" id="c" style="background: #f00"></canvas>
<script>
  window.onload = function () {
    var ctx = document.getElementById('c').getContext('2d');

    ctx.globalCompositeOperation = 'lighter';
    ctx.scale(5, 5);
    ctx.strokeStyle = '#0f0';
    ctx.lineWidth = 10;
    ctx.strokeRect(5, 5, 10, 10);
  }
</script>
